duality
【正确答案】：language is a system, which consists of two sets of structures, or two levels. At the lower level there is a structure of sounds, which are meaningless. But the sounds of language can be grouped into a large number of units of meaning such as morphemes, words, which are found at the higher level of the system.
【题目解析】：语言学是以语言为研究对象的科学，它研究的对象是人类语言，是对语言的科学研究。为了区分人类语言和动物语言，美国语言学家Hockett提出了12大识别特征，其中之一是人类语言的二重性。不同于动物语言，人类语言由两层结构组成，在较低层次的是声音的结构，该声音是无意义的；在较高层次的是意义的结构，该层结构由声音结构的重组而产生，包括语素、词语等。
